In order to prevent decay of wood and timbers, and thereby increasing their life, it is common practice to impregnate the wood or timbers with a preservative such as creosote, mixtures of inorganic compounds dissolved or dispersed in water, or certain organic or metallo-organic compounds which are dissolved in organic solvents. The protection afforded by the application of these materials is dependent upon deep and reasonably uniform penetration into the wood or timber by the preservative material while at the same time leaving a clean surface on the wood-treated product.
The subject of wood treatment and wood preservation is discussed in some detail in the two volume treatise entitled "Wood Deterioration and its Prevention by Preservative Treatments", Darrel D. Nicholas, Editor, Syracuse Wood Science Series 5, Syracuse University Press, Syracuse, N.Y., 1973. Among the examples of wood preservatives described therein are various creosote compositions, pentachlorophenol, copper naphthenate, copper-8-quinolinolate, organotin compounds, organomercury compounds, zinc naphthenate, chlorinated hydrocarbons, ammoniacal copper arsenate (ACA) acid copper chromate (ACC), zinc salts such as zinc chloride, zinc oxide and zinc sulfate, chromated copper arsenate (CCA), etc.
Processes and equipment for treating wood are discussed in Volume II, Chapter 3, pages 279-298. The pressure treatment is described as the most effective method of protecting wood against attack of decay, insects, fire, etc. Non-pressure treatments also are discussed in this chapter. Dipping is suggested primarily as a satisfactory surface treatment although some penetration is observed. Another non-pressure technique is the diffusion process with unseasoned wood. The author indicates the process requires long treating periods because of sloe diffusion rates. Water-solution preservatives are required.
It also has been suggested to improve the method of pressure treatment by first subjecting the wood to a vacuum treatment. Examples of prior art patents describing methods of impregnating wood utilizing a vacuum followed by pressure include U.S. Pat. Nos. 2,668,779; 3,200,003; and 3,968,276.
U.S. Pat. No. 3,677,805 describes a modification of the pressure treatment. In this procedure, the wood is immersed in a treatment liquid inside a pressure vessel, and the pressure is increased to operating pressure whereupon the contents of the vessel then are subjected to the action of a pulsating pump which provides sinusoidal pressure pulses within the vessel. In other words, pressure pulses are applied repetitively in modulated amplitude to provide variable pressure peaks above and below the ambient pressure maintained in the pressure vessel. This procedure requires equipment which includes a pulsating pump operating into a pressure vehicle equipped with a pressure release means.
As mentioned above, the most common commercial procedure for impregnating wood involves subjecting the wood to the preservative under relatively high pressures and sometimes at relatively high temperatures. Normally, the procedure involves placing the wood in a vessel, filling the vessel with the preservative mixture and raising the pressure within the vessel to the desired level to effect penetration of the solution into the wood. Sometimes, the temperature of the liquid within the vessel is raised to an elevated temperature. After the wood has been subjected to the penetrating system for the desired period of time, the pressure is reduced, generally, to atmospheric pressure, and as the pressure is reduced, some of the penetrating solution contained in the wood is forced out of the wood by expansion of the air within the wood as the external pressure is reduced. This penetrating solution which is released and recovered from the wood as the external pressure is reduced is generally referred to in the art as "kickback". When the term is used in this application, it shall have the same meaning.
Sometimes, especially with oil-borne preservatives, before the wood is removed from the vessel, it is subjected to a "post-treatment" to recover additional kickback, or to clean the surface of the final treated product and/or to improve the color of the treated product. Known post-treating procedures include a live steaming process which comprises the steps of introducing live steam into the vessel containing the wood at about 0-10 psig, removing the oily condensate that is formed, applying a vacuum, and finally removing the oil and water vacuum drippings. In another steaming process referred to in the industry as "closed steaming", a large amount of water (at least enough to cover the internal heating coils) is added to the vessel, and the water is heated to the boiling temperature at about 0-10 psig. The hot water is then removed, and a vacuum is applied. Following the vacuum, the vessel is vented to atmospheric pressure and the vacuum drippings of oil and water are removed. In another post-treatment which has been utilized commercially ("vapor drying"), a hydrocarbon or other suitable solvent is added to the vessel containing the wood and the solvent is heated to boiling at an appropriate temperature and pressure. Subsequently, the solvent is removed; a vacuum is applied; and finally the vessel is vented to atmospheric pressure, and the vacuum drippings of the solution of oil-borne preservative in the solvent are removed. In another process referred to as the "expansion bath" process, the preservative-treated wood and the preservative solution are heated to a temperature higher than the temperature used when the pressure was applied to the wood in the presence of the preservative. Generally, this higher temperature is about 210.degree.-220.degree. F., and the temperature is maintained for a given period following which the preservative solution is removed, and a final vacuum is applied to generate additional kickback following which the wood is removed from the vessel.
Some of these post-treatment procedures usually yield kickback as a result of the increase in the temperature. These include the live steaming, closed steaming and expansion bath procedures. The final post-treatment vacuum also normally yields kickback by decreasing the pressure. The vapor drying process obviously requires special procedures and equipment to handle the organic solvents and to prevent fires. In the two steaming post-treatments, an oily water mixture is produced that requires additional special handling.
The above-described prior art represents a sampling of the suggestions which have been made for producing clean treated wood that is dry to the touch when oil-borne preservatives are used. In spite of the many techniques in the prior art, there continues to be a need for a less expensive, safe post-treatment that results in a cleaner surface.